The field of the invention is fault sensing and control of power distribution systems. More particularly, the preferred embodiment of the present invention is directed to reducing the time period of fault sensing and control of impedance insertion, i.e. to provide for impedance insertion in series in the faulted line before fault currents reach unmanageable or destructive amplitudes.
In some power distribution systems, the short circuit current capability has steadily grown to the point that existing station equipment, in particular the circuit breaker is marginal or inadequate in its fault current ratings. The cause of this trend is primarily the continuing increase in power consumption per unit area. However, the problem is compounded by the requirements of system security which have prompted stiff ties to neighboring systems and multiple parallel transmission lines within each system. There are a number of convincing arguments that the growth in short circuit capability occurs at a geometric rate. This factor combined with increased installation costs and lengthened lead times for procurement of new equipment present a strong case for current limiting devices as an alternative to the historic approach of replacement and upgrading of breakers as their ratings are surpassed.
Various devices have been utilized to limit fault current duty. These include resonant L-C links, saturable reactors and at low voltages, static breakers using force-commutated thyristors. The above devices have individual advantages and disadvantages, but all share in common the disadvantage of significant power losses while operating with normal load. In addition, these arrangements utilizing reactive elements tend to be quite bulky and introduce to the system additional problems from transient overvoltages or harmonic currents.
The insertion during the fault of a resistive element in series with a bus or feeder has certain attractive advantages, but requires rapid response and sophisticated sensing and control. In order to be effective, the device should be capable of inserting the current limiting resistor into the transmission line within about one millisecond of the occurrence of the fault. Further, owing to the extensive interconnection common within many systems, operation at transmission voltages, 138 kV and higher, is desirable. Accordingly, rapid response to fault conditions is needed for fast-acting circuit breakers or impedance insertion devices.
Until recently, devices were not available which were capable of both rapid response and operation at sufficiently high voltage to make current limiting by resistor insertion in transmission lines practical. During the past year, tests have demonstrated the practicality of interrupting high voltage direct current (HVDC) lines at the 100 kilovolt, (kV), 1 kiloampere (kA) level in times as short as two milliseconds. Tests on improved models of this apparatus have been carried forward to 5 kA, still retaining the 100 kV modular unit size. In addition, opening times as short as one millisecond have been achieved on prototype mechanical switches suitable to the current and voltage levels required. Power distribution systems providing current limiting by resistor insertion of the preferred type are disclosed in U.S. Pat. Nos. 3,611,031, 3,641,358, 3,660,723, 3,657,607, 3,777,179, 3,781,606, and Re. 27,557 having a common assignee and are incorporated here by reference; and preferred current interrupters of the crossedfield switch tube type which can be turned off without reducing current to zero and thereby can transfer the current into a parallel resistor are cited infra.
Resistor insertion type ac current limiting devices rated for 138 kV, 10 kA interrupt capacity and based upon the foregoing breaker are disclosed in copending application of the present inventor and William L. Dugan, Ser. No. 488,345 filed July 15, 1974, entitled "Impedance Sensing System and In Line Device Therefor" assigned to the common assignee and incorporated herein by reference.
Prior methods of fault sensing, e.g. measuring rms value of the fault current, require a long sensing period including several periods of an alternating current (ac) cycle which is on the order of tens of milliseconds (ms). The prior methods are suitable for conventional circuit breakers that require at least two cycles for contact opening and arc interruption of the breaker but are not suitable for fast-acting breakers or impedance insertion devices.
In the prior method of fault sensing, the circuit breaker is tripped when the measured rms value exceeds a predetermined critical level. Obviously, sensing in a shorter time period than the time interval of an ac cycle cannot be accomplished directly and the rms value must be inferred by projection of an initial fault current build-up. A further complication in this procedure of projection is that faults can start at any time within the ac cycle.
Recent developments in electrical power breakers include one-cycle and synchronous breakers and also current limiting breakers. These recently developed breakers are dependent upon very short time intervals for fault sensing to utilize their improved capabilities, i.e. sensing time intervals of less than a millisecond to a few milliseconds.